1. Field of the Invention
The present invention relates to a radio frequency tuner. Such a tuner may be used, for example, to receive and select signals from two or more radio frequency sources.
2. Description of the Related Art
FIG. 1 of the accompanying drawings illustrates a tuner for receiving signals from a cable transmission network and a terrestrial transmission network and for selecting a channel from either network for reception. Tuners of this type are known as “Plug and Play”. Such a tuner is connected to two (or more) networks at the same time. Both networks contain channels within a frequency range which is typically from approximately 47 MHz to approximately 1100 MHz. It is also typical for the channel frequency allocation to be substantially identical between the networks such that the channels of one network occupy the same frequencies as the channels of the other network.
In the case of a cable distribution network, the received spectrum is generally heavily populated by channels and is controlled so as to deliver a power range of between −64 and −34 dBm to a consumer. In the case of a terrestrial distribution network, fewer channels are typically present. However, the channel power range is generally not as well controlled as in the case of a cable distribution network so that channel power may vary from the thermal noise floor to, for example, 0 dBm. In general, at least some of the channel frequency allocations occupied in one of the networks will also be occupied in the other of the networks and sufficient isolation must be provided in order to avoid a potentially interfering channel from one network appearing on a selected channel from the other network. In particular, sufficient isolation must be present to prevent interference with reception by the tuner and to prevent contamination of network signals distributed to other consumers.
As shown in FIG. 1, an enclosure 11, for example in the form of a Faraday cage or “tin box”, has radio frequency inputs 12 and 13 for connection to different distribution networks containing channels with the same frequency allocations in the same frequency ranges. The inputs 12 and 13 are connected to respective inputs of a radio frequency (RF) relay 14, which is controlled so as to act as a single pole changeover electronic switch for selectively connecting either of the distribution networks to the input of a tuner 15. The tuner 15 may be of any suitable architecture for converting any selected input channel to a desired intermediate frequency and for supplying this to the tuner output 16.
The RF relay 14 is a relatively expensive component which has to be physically large in order to provide the physical separation which is necessary for the required isolation between the distribution networks connected to the inputs 12 and 13 and in order to provide sufficient isolation between the non-selected relay input and the relay output. Also, the presence of the relay 14 in the signal path to the input of the tuner 15 results in an insertion loss which degrades the noise figure achievable by the tuner 15.
Although a typical example of the relay 14 can achieve isolation of the order of 80 dB at lower frequencies within the frequency range of the distribution networks, the isolation reduces with increasing frequency and so results in a degradation in the carrier-to-noise performance. In practice, although the resulting performance at higher frequencies is not ideal, it can be made acceptable.
Another disadvantage of the relay 14 is that, because it is a mechanical component, it exhibits degradation with time caused by repeated switching. Such degradation may include, for example, an increase in insertion loss caused by the build-up of contact resistance. This effect results in a reduction in noise figure and a degradation in impedance matching. Further, because of increased levels of surface contaminants on the relay contacts, the intermodulation performance may become degraded with time as such contaminants can have semiconducting properties resulting in non-linear distortion generation. Such degrading effects may be further compounded by the temperature variations which are typically experienced in a “set top box” application of such a tuner.
A further disadvantage of the use of an RF relay is that devices of this type are generally only available as changeover or two-way switches and can therefore only switch between two distribution networks. If more than two distribution networks are present, then further such relays have to be provided. For example, as shown in FIG. 2 of the accompanying drawings, an additional relay 27 has to be provided in order to provide selection from among three distribution networks connected to respective inputs 22, 23 and 28. Apart from the additional cost and bulk of such further relays, some of the signals are subjected to the cumulative effects of having passed through several relays. For example, in the three input case illustrated in FIG. 2, signals from the inputs 22 and 23 have to pass through both relays 24 and 27 in order to reach the tuner 25.